The present invention relates to the field of electron sources for use in electron beam applications, and in particular to Schottky emitters.
Electron emission cathodes, typically referred to as electron sources, are used in devices such as scanning electron microscopes, transmission electron microscopes, semiconductor inspection systems, and electron beam lithography systems. In such devices, an electron source provides electrons, which are then guided into an intense, finely focused beam of electrons having energies within a narrow range. To facilitate formation of such a beam, the electron source should emit a large number of electrons within a narrow energy band. The electrons should be emitted from a small surface area on the source into a narrow cone of emission. Electron sources can be characterized by a brightness, which is defined as the electron current divided by the real or virtual product oft he emission area and the solid angle through which the electrons are emitted. A practical source should be bright and should operate for an extended period of time with little or no maintenance and minimal noise, that is, variations in the amount and energy of the emitted electrons.
Electrons are normally prevented from leaving the atoms at the surface of an object by an energy barrier. The amount of energy required to overcome the energy barrier is known as the xe2x80x9cwork functionxe2x80x9d of the surface. One type of electron source, a thermionic emission source, replies primarily on heat to provide the energy to overcome the energy barrier and emit electrons. Thermionic emission sources are not sufficiently bright for use in many applications.
Another type of electron source, a cold field emission source, operates at room temperature and relies on a strong electric field to facilitate the emission of electrons by tunneling through the energy barrier. A field electron source typically includes a narrow tip at which electrons leave the surface and are ejected into the surrounding vacuum. While cold field emission sources are much smaller and brighter than thermionic emission sources, cold field emission sources exhibit instabilities that cause problems in many applications.
Yet another type of electron source is referred to as a Schottky emission cathode or Schottky emitter. Although the term xe2x80x9cSchottky emissionxe2x80x9d refers to a specific operating mode of an emitter, the term xe2x80x9cSchottky emitterxe2x80x9d is used more broadly to describe a type of electron emitter that may be capable of operating in a variety of modes, including Schottky emission mode. Schottky emitters use a coating on a heated emitter tip to reduce its work function. The coating typically comprises a very thin layer, such as a fraction of a monolayer, of an active metal. In Schottky emission mode, a Schottky emitter uses a combination of heat and electric field to emit electrons, which appear to radiate from a virtual point source within the tip. With changes to the emitter temperature and electric field, the Schottky emitter will emit in other emission modes or combinations of emission mode, including extended Schottky emission mode and thermal field mode. Schottky emitters are very bright and are more stable and easier to handle than cold field emitters. Because of their performance and reliability benefits, Schottky emitters have become a common electron source for modem focused electron beam systems.
FIG. 1 shows part of a typical prior art Schottky emitter 12, such as the one described in U.S. Pat. No. 3,814,975 to Wolfe et al. for xe2x80x9cElectron Emission System.xe2x80x9d Schottky emitter 12 includes a filament 14 that supports and heats an emitter 16 having an apex 22 from which the electrons are emitted. Applicants herein use the term xe2x80x9cemitterxe2x80x9d alone to refer to that portion of the electron source from which electrons are emitted (e.g., emitter 16 of FIG. 1) and the term xe2x80x9cSchottky emitterxe2x80x9d refers to the entire electron source assembly (e.g., Schottky emitter 12), often including a suppressor cap described below. Heating current is supplied to filament 14 through electrodes 24 that penetrate a base 26. Schottky emitter 12 typically operates with apex 22 at a temperature of approximately 1,800 K. Emitter 16 is typically made from a single crystal of tungsten oriented in the  less than 100 greater than ,  less than 110 greater than ,  less than 111 greater than , or  less than 310 greater than  direction. Emitter 16 could also be made of other materials, such as molybdenum, iridium, or rhenium. Emitter 16 is coated with a coating material to lower its work function. Such coating materials could include, for example, compounds, such as oxide, nitrides and carbon compounds, of zirconium, titanium, hafnium, yttrium, niobium, vanadium, thorium, scandium, beryllium or lanthanum. For example, coating a (100) surface of tungsten with zirconium and oxygen lowers the work function of the surface from 4.5 eV to 2.8 eV. By reducing the energy required to emit electrons, the coating on the emitter makes it a brighter electron source.
At the high temperatures at which Schottky emitter 12 operates, the coating material tends to evaporate from emitter 16 and must be continually replenished to maintain the low work function at apex 22. A reservoir 28 of the coating material is typically provided to replenish the coating on emitter 16. The material from reservoir 28 diffuses along the surface and through the bulk of emitter 16 toward apex 22, thereby continually replenishing the coating there. Schottky emitter 12 includes a reservoir 28 of coating material positioned at the junction of emitter 16 and filament 14. Methods for coating emitters and fabricating reservoirs of coating materials are known. For example, reservoir 28 may be formed by adding a powder of a precursor material, such as zirconium hydride, to a solvent, such as water or isoamyl acetate, to make a slurry and then adhering the slurry to the emitter 16. When the emitter is heated, the zirconium hydride decomposes into zirconium and hydrogen, which evolves off. The emitter 16 is then heated in an atmosphere of oxygen to form a zirconium oxide coating and reservoir. It will be understood that the term zirconium oxide is used to indicate any combination of zirconium and oxygen atoms and is not limited to any particular atomic ratio.
At the high operating temperatures of the Schottky emitter 12, not only does the coating material on emitter 16 and apex 22 evaporate, the coating material also evaporates directly from the reservoir, depleting it. The evaporation rate of the coating material in the reservoir increases exponentially with the temperature. Thus, the useful life of the reservoir depends upon the amount of material in the reservoir and its temperature. At a constant temperature, increasing the mass of the reservoir increases its life. Large increases in reservoir mass are not practical, however, because the coating material in a large reservoir tends to separate from the emitter, reducing the reservoir mass and causing problems in the vacuum system.
When reservoir 28 is depleted, Schottky emitter 12 no longer functions properly, and it is necessary to shut down the electron beam system in which Schottky emitter 12 is installed to replace the emitter. Because such electron beam systems are often critical links in the manufacturing of complex integrated circuits, shutting down a system can delay production and is therefore costly. It is desirable, therefore, to extend the life of the reservoir as much as possible, thereby extending the life of the emitter.
FIG. 2 shows a part of another prior art Schottky emitter 34, similar to the one described in J. E. Wolfe, xe2x80x9cOperational Experience with Zirconiated T-F Emitters,xe2x80x9d J. Vac. Sci. Tech. 16(6) (1979) and U.S. Pat. No. 5,449,968 to Terui for xe2x80x9cThermal Field Emission Cathode.xe2x80x9d FIG. 2 shows an emitter 36 connected to a filament 38 at a junction 44 and terminating in an apex 46. (Emitter 12 of FIG. 1 also included a junction, but it was hidden by reservoir 28.) Because heat is supplied to emitter 36 from filament 38, the emitter 36 is hottest at junction 44 and is cooler as the distance from junction 44 increases. Schottky emitter 34 includes a reservoir 50 positioned away from junction 44 towards apex 46. Positioning reservoir 50 away from junction 44 allows the reservoir 50 to remain cooler during operation, thereby reducing evaporation of the coating material and increasing the useful life of the emitter. However, positioning reservoir 50 too close to apex 46 adversely affects the electric field used to pull electrons from apex 46. According to U.S. Pat. No. 5,449,968, the optimum position for the reservoir is at approximately 200 xcexcm away from junction 44 toward apex 46.
At such a position, reservoir 50, though cooler than junction 44, is still hotter than apex 22. Evaporation still limits the life of reservoir 50, and its lifetime is still the limiting factor oft he useful life of Schottky emitter 34.
An object of the invention is, therefore, to provide an electron emitter having an extended useful life.
Another object of the invention is to provide a longer lasting reservoir for an electron emitter.
Still another object is to provide a method of manufacturing an electron emitter having an extended life.
Yet another object of the invention is to increase the reliability of electron beam systems such as electron microscopes.
Still a further object oft he invention is to provide an electron beam system requiring reduced maintenance due to improved electron source lifetime.
The invention comprises a an electron emitter, preferably a Schottky emitter, having an extended useful life, a method of manufacturing the electron emitter, a method of providing electrons for an electron beam system, and an electron beam system using the electron emitter. In accordance with the invention, an electron emitter includes an emitter and a filament attached to the emitter at a junction. The emitter extends forward from the junction and terminates in an apex from which electrons are emitted. The emitter also extends rearward from the junction, and a reservoir of material for coating the emitter is positioned on the portion of the emitter extending rearward from the junction.
Applicants have discovered that an adequate coating is maintained at the emitter apex when the reservoir is positioned on the opposite side of the junction from the apex, even though the coating material must diffuse through a greater distance to reach the apex and the diffusion path is across the junction, which is the hottest part of the emitter. By positioning the reservoir on the rearward-extending portion of the emitter, the distance between the reservoir and the junction is not limited by the distance between the junction and the apex, and the reservoir can be positioned far from the junction without adversely affecting the electric field at the apex. By positioning the reservoir further from the junction, the reservoir is maintained at a lower temperature than in prior art emitters, in which the reservoir is at a temperature typically less than that of the junction and greater than that of the apex. The coating material in the reservoir of the present invention evaporates more slowly, greatly improving the useful life of the emitter. In some embodiments, the reservoir is positioned at a distance greater than or equal to the distance from the junction to the apex, and the reservoir can be maintained at a temperature lower than that of the apex.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages oft he invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.